1. Field of the Invention
This invention relates generally to the field of geophysical prospecting. More particularly, the invention relates to the field of marine geophysical surveys.
2. Description of the Related Art
In the oil and gas industry, geophysical prospecting is commonly used to aid in the search for and evaluation of subterranean formations. Geophysical prospecting techniques yield knowledge of the subsurface structure of the earth, which is useful for finding and extracting valuable mineral resources, particularly hydrocarbon deposits such as oil and natural gas. Well-known techniques of geophysical prospecting include seismic and electromagnet surveys. The resulting geophysical data are processed to yield information relating to the geologic structure and properties of the subterranean formations in the area being surveyed.
In a marine seismic survey, seismic energy sources are used to generate a seismic signal which, after propagating into the earth, is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, specifically sound wave velocity and rock density, which lead to differences in acoustic impedance at the interfaces. The reflected seismic energy is detected by seismic sensors (also called seismic receivers) and recorded.
The appropriate seismic sources for generating the seismic signal in marine seismic surveys typically include a submerged seismic source towed by a ship and periodically activated to generate an acoustic wavefield. The seismic source generating the wavefield is typically an air gun or a spatially-distributed array of air guns.
The appropriate types of seismic sensors typically include particle velocity sensors (typically, geophones) and water pressure sensors (typically, hydrophones) mounted within a towed seismic streamer (also known as a seismic cable). Seismic sensors are commonly deployed in sensor arrays within the streamer.
Seismic sources, seismic streamers, and other attached equipment are towed behind survey vessels, attached by cables. The seismic sources and seismic streamers may be positioned in the water by attached equipment, such as deflectors and cable positioning devices (also known as “birds”).
Another technique of geophysical prospecting is an electromagnetic survey. Electromagnetic sources and receivers include electric sources plus magnetic sources and receivers. The electric and magnetic receivers can include multi-component receivers to detect horizontal and vertical electric signal components and horizontal and vertical magnetic signal components. In some electromagnetic surveys, the sources and receivers are towed through the water, possibly along with other equipment. The electromagnetic sources and receivers are sometimes towed in streamers, as described above for the seismic receivers.
Unfortunately, marine organisms adhere to nearly everything that moves through water for significant periods of time, including towed geophysical equipment. This adherence of marine organisms is known as “bio-fouling”. Bio-fouling is conventionally conceived of in terms of barnacles, but also includes the growth of mussels, oysters, algae, tubeworms, slime, and other marine organisms.
Thus, a need exists for a system and a method for protecting towed geophysical equipment in marine geophysical surveys, especially towed streamers and equipment attached thereto, from marine growth. A need also exists for a system and method for reducing drag resistance on the towed geophysical equipment.